1. Field of the Invention
The present invention relates to a rotary table device for a machine tool including a main shaft that is rotatably supported in an insertion hole in a frame, a table that is secured to an end side of the main shaft and on which a workpiece, which serves as an object to be processed, is mounted, and a clamp piston that is provided so as to be movable in an axial direction of the main shaft in the insertion hole.
2. Description of the Related Art
As such a rotary table device for a machine tool described above, a rotary table device disclosed in Japanese Unexamined Utility Model Registration Application Publication No. 03-120354 (Patent Literature (PTL) 1) is provided. More specifically, PTL 1 discloses a rotary table device including a main shaft (output shaft) that is rotatably supported in an insertion hole (void) in a frame (housing), a table (indexing table) that is secured to an end side of the main shaft and on which a workpiece, which serves as an object to be processed, is mounted, and a clamp piston (cylinder) that is movable in an axial direction in the insertion hole.
In the rotary table device disclosed PTL 1, the main shaft has a clamp surface (friction surface) facing the table; the clamp piston and a member (piston) secured to the frame define a pressure chamber; and, when an operating fluid is supplied to the pressure chamber, the clamp piston is caused to press-contact the clamp surface, so that the table is brought into a clamped state. In the rotary table device in PTL 1, a bearing is provided between the frame and the main shaft, and the bearing rotatably supports the table through the main shaft.
As such a rotary table device, there is a type in which a workpiece that is mounted on the table while the workpiece is placed on the table is processed by a machine tool. Types of such processing include what is called positioning processing in which the table is brought into a clamped state at an indexed angular position, and what is called contouring processing in which a workpiece is processed while the table is rotated without bringing the workpiece into a clamped state (that is, the table is brought into an unclamped state). In the rotary table device, the positioning processing and the contouring processing may be successively performed on the mounted workpiece. In other words, it may be necessary for one rotary table device to be formed so as to be capable of performing both the positioning processing and the contouring processing.
In the rotary table device disclosed in PTL 1, there is a problem in such processing, in particular, the positioning processing, performed on a workpiece in that the processing precision thereof is reduced to an unallowable precision. This is described in more detail below.
Ordinarily, when processing a workpiece mounted on the table of the rotary table device, an outside force (cutting force) generated by a tool or the like acts upon the workpiece. Therefore, in the rotary table device, due to the cutting force that acts upon the workpiece when the processing is performed, a moment around an axis that crosses a rotation axis of the table acts upon a portion that supports the table (supporting structure). However, since, in general, in the contouring processing, light cutting is often performed as finishing processing or the like, the cutting force that acts upon the workpiece when the contouring processing is performed is relatively small, so that the moment that acts upon the supporting structure is relatively small.
However, in the positioning processing, heavy cutting, such as rough processing or the like, is often performed. Therefore, the cutting force that acts upon the workpiece when the positioning processing is performed is larger than that when the contouring processing is performed. Consequently, when the positioning processing is performed, the moment that acts upon the supporting structure of the table is large.
For this case, as described above, in the rotary table device in PTL 1, a bearing is provided between the frame and the main shaft, and a supporting structure is one in which the table is indirectly supported by the frame through the main shaft. Therefore, in the positioning processing, such a large moment mentioned above acts upon the supporting structure, as a result of which the main shaft is flexed. The flexing of the main shaft may cause the table to be brought into a tilted state. Such tilting of the table may cause the precision with which the workpiece is processed to be reduced to an unallowable precision.
As such a rotary table device for a machine tool described above, a rotary table device having a structure disclosed in Japanese Unexamined Patent Application Publication No. 2005-138216 (Patent Literature (PTL) 2) is provided. In the rotary table device disclosed in PTL 2, a bearing for supporting a table is provided directly below the table and between the table and a frame. That is, in the rotary table device in PTL 2, a supporting structure for supporting the table is one in which the table is directly supported by the frame. Therefore, in the rotary table device in PTL 2, when the positioning processing is performed, a state in which the table is tilted by flexing of a main shaft, such as that described above, does not occur.
The bearing is disposed in the rotary table device in PTL 2 as described above on the assumption that a structure for bringing the table into a clamped state (clamp structure) differs from that of the rotary table device in PTL 1. That is, in the rotary table device in PTL 2, the table is provided so as to be movable in the axial direction of the main shaft, and a clamp surface is formed at a lower surface of the table. In causing the table to be in a clamped state, the table is displaced towards the frame, and the lower surface (clamp surface) of the table is caused to press-contact the frame. In PTL 2, with the rotary table device being assumed as having such a clamp structure, when rotationally driving the table (that is, unclamping the table), for preventing the table and the frame from slide-contacting each other, the clamp surface of the table is set away from the frame and, in this state, the bearing is disposed as described above such that the table is easily rotated.
Incidentally, the rotary table device in PTL 2 is basically formed only for performing positioning processing, that is, the rotary table device in PTL 2 is not formed for performing contouring processing such as that described above. More specifically, the rotary table device in PTL 2 has a structure in which, as described above, the unclamped table is displaced towards the frame to bring the table into a clamped state. In the axial direction of the main shaft, the position of the upper surface of the table (that is, the position of a workpiece) when the table is clamped differs from that when the table is unclamped. Therefore, when one tries to perform contouring processing with the rotary table device having such a structure, the processing standard when the contouring processing is performed differs from that when positioning processing is performed. Therefore, processing control (NC control) becomes complicated.
Moreover, in the rotary table device in PTL 2, in order for the table to be displaceable as described above, in addition to the bearing, an urging device, such as a compression coil spring, is interposed between the table and the frame. When rotationally driving the table (that is, unclamping the table), the table is supported by the frame with the urging device interposed therebetween. That is, in the rotary table device in PTL 2, when the table is in an unclamped state in which the table is rotationally driven, the table is elastically supported by the frame. Therefore, when contouring processing is performed in the rotary table device having such a structure, the table is tilted by the cutting force acting upon the table through the workpiece, as a result of which the precision with which the workpiece is processed is considerably reduced. Therefore, due to these points, it can be said that the rotary table device in PTL 2 is formed only for positioning processing.